The present invention is related to angiography and, in particular, the invention relates to a system and method for producing angiographic images using x-ray projection data, such as may be acquired using a computed tomography (CT) system.
In a computed tomography system, an x-ray source projects a fan-shaped beam which is collimated to lie within an X-Y plane of a Cartesian coordinate system, termed the “image plane.” The x-ray beam passes through the object being imaged, such as a medical patient, and impinges upon an array of radiation detectors. The intensity of the transmitted radiation is dependent upon the attenuation of the x-ray beam by the object and each detector produces a separate electrical signal that is a measurement of the beam attenuation. The attenuation measurements from all the detectors are acquired separately to produce what is called the “transmission profile,” or “attenuation profile” or “projection.”
The source and detector array in a conventional CT system are rotated on a gantry within the imaging plane and around the object so that the angle at which the x-ray beam intersects the object constantly changes. The transmission profile from the detector array at a given angle is referred to as a “view” and a “scan” of the object comprises a set of views made at different angular orientations during one revolution of the x-ray source and detector. In a 2D scan, data is processed to construct an image that corresponds to a two dimensional slice taken through the object. The prevailing method for reconstructing an image from 2D data is referred to in the art as the filtered backprojection technique. This image reconstruction process converts the attenuation measurements acquired during a scan into integers called “CT numbers” or “Hounsfield units”, which are used to control the brightness of a corresponding pixel on a display.
The filtered backprojection image reconstruction method is the most common technique used to reconstruct CT images from acquired transmission profiles. As shown in FIG. 1A each acquired x-ray transmission profile 100 is backprojected onto the field of view (FOV) 102 by projecting each ray sum 104 in the profile 100 through the FOV 102 along the same ray path that produced the ray sum 104 as indicated by arrows 106. In projecting each ray sum 104 in the FOV 102 we have no a priori knowledge of the subject and the assumption is made that the x-ray attenuation in the FOV 102 is homogeneous and that the ray sum should be distributed equally in each pixel through which the ray path passes. For example, a ray path 108 is illustrated in FIG. 1A for a single ray sum 104 in one transmission profile 100 and it passes through N pixels in the FOV 102. The attenuation value (P) of this ray sum 104 is divided up equally between these N pixels:μn=(P×1)/N  (1);
where: μn is the attenuation value distributed to the nth pixel in a ray path having N pixels.
Clearly, the assumption that attenuation in the FOV 102 is homogeneous is not correct. However, as is well known in the art, if certain corrections are made to each transmission profile 100 and a sufficient number of profiles are acquired at a corresponding number of projection angles, the errors caused by this faulty assumption are minimized and image artifacts are suppressed. In a typical filtered backprojection method of image reconstruction, anywhere from 400 to 1000 views are typically required to adequately suppress image artifacts in a 2D CT image.
There are a number of clinical applications where the time required to acquire a large number of views is not available. In time-resolved angiography, for example, a series of images are acquired as contrast agent flows into the region of interest. Each image is acquired as rapidly as possible to obtain a series of snapshots that depicts the in-flow of contrast. This application is particularly challenging when imaging coronary arteries or other vessels that require cardiac gating to suppress motion artifacts.
Since the introduction of angiography beginning with the direct carotid artery punctures of Moniz in 1927, there have been ongoing attempts to develop angiographic techniques that provide diagnostic images of the vasculature, while simultaneously reducing the invasiveness associated with the procedure. For decades, post-processing of images was largely limited to the use of film subtraction techniques. Initial angiographic techniques involved direct arterial punctures and the manipulation of a needle through which a contrast medium was injected. These practices were associated with a significant incidence of serious complications. The development of percutaneous techniques allowing the use of a single catheter to study multiple arterial segments reduced, but this by no means eliminated, these adverse events. In the late 1970's, a technique known as digital subtraction angiography (DSA) was developed based on real-time digital processing equipment. Because of the advantages of digital processing, it was originally hoped that DSA could be consistently implemented using an IV injection of contrast medium, thus reducing both the discomfort and the incidence of complications associated with direct IA injections.
However, it quickly became apparent that the IV-DSA technique was limited by problems due to suboptimal viewing angles and vessel overlap that could only be reduced by repeated injections. Even then, these factors were problematic unless a projection that avoided the overlap of relevant vascular structures could be defined. Similar problems occurred when using biplane acquisitions. Also, because of the limited amount of signal associated with the IV injection of contrast medium, IV-DSA was best performed in conditions with adequate cardiac output and minimal patient motion. IV-DSA was consequently replaced by techniques that combined similar digital processing with standard IA angiographic examinations. Nevertheless, because DSA can significantly reduce both the time necessary to perform an angiographic examination and the amount of contrast medium that was required, its availability resulted in a significant reduction in the adverse events associated with angiography. Due to steady advancements in both hardware and software, DSA can now provide exquisite depictions of the vasculature in both 2D and rotational 3D formats. Three-dimensional digital subtraction angiography (3D-DSA) has become an important component in the diagnosis and management of people with a large variety of central nervous system vascular diseases.
Current limitations in the temporal resolution capabilities of x-ray angiographic equipment require that rotational acquisitions be obtained over a minimum time of about 5 seconds. Even with perfect timing of an acquisition so that arterial structures are fully opacified at the onset of a rotation, there is almost always some filling of venous structures by the end of the rotation. Display of a “pure” image of arterial anatomy is only achieved by thresholding such that venous structures, which contain lower concentrations of contrast medium than arterial structures, are no longer apparent in the image. This limitation is a significant factor in making it prohibitively difficult to accurately measure the dimensions of both normal and abnormal vascular structures. Traditional DSA-based techniques do not depict the temporal sequence of filling in a reconstructed 3D-DSA volume.
In recent years competition for traditional DSA has emerged in the form of CT angiography (CTA) and Magnetic Resonance Angiography (MRA). CTA provides high spatial resolution, but is not time-resolved unless the imaging volume is severely limited. CTA is also limited as a standalone diagnostic modality by artifacts caused by bone at the skull base and the contamination of arterial images with opacified venous structures. Further, CTA provides no functionality for guiding or monitoring minimally-invasive endovascular interventions. Significant advances have been made in both the spatial and the temporal resolution qualities of MRA. Currently, gadolinium-enhanced time-resolved MRA (TRICKS) is widely viewed as a dominant clinical standard for time-resolved MRA. TRICKS enables voxel sizes of about 10 mm3 and a temporal resolution of approximately 10 seconds. Advancements such as HYBRID HYPR MRA techniques, which violate the Nyquist theorem by factors approaching 1000, can provide images with sub-millimeter isotropic resolution at frame times just under 1 second. Nonetheless, the spatial and temporal resolution of MRA are not adequate for all imaging situations and its costs are considerable.
Therefore, it would be desirable to have a system and method that improves the available information related to angiography, such as by providing quantitative information.